1. Field of the Invention
The present invention relates in general to an electro-optical device such as a liquid crystal display and a method of driving an electro-optical device such as a liquid crystal display.
2. Description of the Prior Art
There are known two types of liquid crystal displays comprising pixels in matrices, i.e. the simple matrix type and the active matrix type. The former type can be designed in simple structure with large aperture ratios and therefore the production cost is relatively low due to few technical constraints on the manufacturing processes. The contrast of images displayed, however, is not so high because of the dynamic operation. In addition to this, when a nematic liquid crystal material is utilized as the liquid crystal medium, a cross-talking phenomena occurs between adjacent pixels.
Contrary to this, in the case of the liquid crystal display of the active matrix type, cross-talking phenomena are effectively suppressed since the operation of the respective pixels are controlled by active devices such as diodes or transistors. The yield in the manufacture, however, is not so high and there are many constraints on the process because the active devices must be provided for respective pixels, e.g. two devices per one pixel.
Ferroelectric liquid crystal displays or anti-ferroelectric liquid crystal displays can display clear static images with high contrasts by making use of their memory performance. Furthermore, cross-talking is unlikely since the optical characteristics of ferroelectric liquid crystal materials or anti-ferroelectric liquid crystal materials are not modified in accordance with effective voltages unlike in the case of nematic liquid crystal materials. Still further, the response speeds of ferroelectric liquid crystal materials or anti-ferroelectric liquid crystal materials are as high as 10 microseconds or higher, which speeds are higher than those of nematic liquid crystal materials by orders of magnitude.
The high memory performance of ferroelectric liquid crystal materials or anti-ferroelectric liquid crystal materials, however, serves to cause the so-called "afterimage" when a displayed image is maintained for a long time, because of a certain type of degradation of the liquid crystal materials. 0f course, if the liquid crystal material having a high memory performance is replaced by another liquid crystal material having a low memory performance in order to avoid the undesirable "afterimage", it is difficult to display clear static images with high contrasts. The image becomes unsightly particularly at a high temperature in this case.
The applicants have proposed a new type of liquid crystal display capable of displaying clear static images with high contrasts by combining a ferroelectric liquid crystal or an anti-ferroelectric liquid crystal and an organic ferroelectric material, e.g. the copolymer of vinylidene fluoride and trifluoroethylene. This technique has been described in Japanese Patent Application No. sho 61-1152.
It is difficult, however, to display grayscale images by the new type display since the optical characteristics of ferroelectric liquid crystal materials or anti-ferroelectric liquid crystal materials are not modified in accordance with effective voltages. Because of this, shades of gray are accomplished only in accordance with the area adjustment technique or the frame control technique. In accordance with the former technique, one pixel has to comprise a plurality of sub-pixels for making it possible to display images in many shades of gray so that the process becomes substantially complicated to require a high cost. In accordance with the later technique, it is likely that flickering may occur.